Some more obvious differences:
G-2 - basic Gustav
G-6 - MG131 cowl bulges, main wheel bulges on wings, non-retractable tail wheel
G-6(late) - same as G-6, but with "Galland hood" canopy
G-6AS/ASM,G-10,G14 - MG-131 bulges are more streamlined, other things are pretty much same, mosty different antenna mast is in question
K-4 - retractable tail wheel again, different antenna mast(not sure but AFAIK it is)
Those are just some main visual differences without checking my WW2 airplane library, performance differences are mostly engine related although streamlined MG131 cowl bulges and retractable tail wheel on the K-4 gave for a less draggy aircraft making K-4 one of the fastest WW2 propeller driven planes...